Transversely-excited film bulk acoustic resonator matrix filters with split die sub-filters

ABSTRACT

A radio frequency filter includes at least a first sub-filter and a second sub-filter connected in parallel between a first port and a second port. Each of the sub-filters has a piezoelectric plate having front and back surfaces, the back surface attached to a substrate, and portions of the piezoelectric plate forming diaphragms spanning respective cavities in the substrate. A conductor pattern is formed on the front surface of the plate, the conductor pattern includes interdigital transducers (IDTs) of a respective plurality of resonators, with interleaved fingers of each IDT disposed on a respective diaphragm of the plurality of diaphragms. A thickness of the portions of the piezoelectric plate of the first sub-filter is different from a thickness of the portions of the piezoelectric plate of the second sub-filter.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever.

RELATED APPLICATION INFORMATION

This patent is a continuation of co-pending U.S. patent application Ser. No. 17/362,727, filed Jun. 29, 2021, titled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR MATRIX FILTERS WITH SPLIT DIE SUB-FILTERS, which claims priority to provisional patent application 63/127,095, filed Dec. 17, 2020, titled SPLIT SUB-FILTER MATRIX XBAR FILTER and is a continuation-in-part of U.S. patent application Ser. No. 17/133,849, filed Dec. 24, 2020, titled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR MATRIX FILTERS, which is a continuation-in-part of application Ser. No. 17/121,724, filed Dec. 14, 2020, titled ACOUSTIC MATRIX FILTERS AND RADIOS USING ACOUSTIC MATRIX FILTERS, which claims priority from provisional patent application 63/087,789, filed Oct. 5, 2020, entitled MATRIX XBAR FILTER. All of these applications are incorporated herein by reference.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic wave resonators, and specifically to filters for use in communications equipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to pass some frequencies and to stop other frequencies, where “pass” means transmit with relatively low signal loss and “stop” means block or substantially attenuate. The range of frequencies passed by a filter is referred to as the “pass-band” of the filter. The range of frequencies stopped by such a filter is referred to as the “stop-band” of the filter. A typical RF filter has at least one pass-band and at least one stop-band. Specific requirements on a passband or stop-band depend on the specific application. For example, a “pass-band” may be defined as a frequency range where the insertion loss of a filter is better than a defined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be defined as a frequency range where the rejection of a filter is greater than a defined value such as 20 dB, 30 dB, 40 dB, or greater depending on application.

RF filters are used in communications systems where information is transmitted over wireless links. For example, RF filters may be found in the RF front-ends of cellular base stations, mobile telephone and computing devices, satellite transceivers and ground stations, IoT (Internet of Things) devices, laptop computers and tablets, fixed point radio links, and other communications systems. RF filters are also used in radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for each specific application, the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size and cost. Specific design and manufacturing methods and enhancements can benefit simultaneously one or several of these requirements.

Performance enhancements to the RF filters in a wireless system can have broad impact to system performance. Improvements in RF filters can be leveraged to provide system performance improvements such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements can be realized at many levels of the wireless system both separately and in combination, for example at the RF module, RF transceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonly incorporate acoustic wave resonators including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic wave resonators (FBAR), and other types of acoustic resonators. However, these existing technologies are not well-suited for use at the higher frequencies and bandwidths proposed for future communications networks.

The desire for wider communication channel bandwidths will inevitably lead to the use of higher frequency communications bands. Radio access technology for mobile telephone networks has been standardized by the 3GPP (3^(rd) Generation Partnership Project). Radio access technology for 5^(th) generation mobile networks is defined in the 5G NR (new radio) standard. The 5G NR standard defines several new communications bands. Two of these new communications bands are n77, which uses the frequency range from 3300 MHz to 4200 MHz, and n79, which uses the frequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79 use time-division duplexing (TDD), such that a communications device operating in band n77 and/or band n79 use the same frequencies for both uplink and downlink transmissions. Bandpass filters for bands n77 and n79 must be capable of handling the transmit power of the communications device. WiFi bands at 5 GHz and 6 GHz also require high frequency and wide bandwidth. The 5G NR standard also defines millimeter wave communication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is an acoustic resonator structure for use in microwave filters. The XBAR is described in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR, which is incorporated herein by reference. An XBAR resonator comprises an interdigital transducer (IDT) formed on a thin floating layer, or diaphragm, of a single-crystal piezoelectric material. The IDT includes a first set of parallel fingers, extending from a first busbar and a second set of parallel fingers extending from a second busbar. The first and second sets of parallel fingers are interleaved. A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm. XBAR resonators provide very high electromechanical coupling and high frequency capability. XBAR resonators may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. XBARs are well suited for use in filters for communications bands with frequencies above 3 GHz. Matrix XBAR filters are also suited for frequencies between 1 GHz and 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view, two schematic cross-sectional views, and a detailed cross-sectional view of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2A is an equivalent circuit model of an acoustic resonator.

FIG. 2B is a graph of the admittance of an ideal acoustic resonator.

FIG. 2C is a circuit symbol for an acoustic resonator.

FIG. 3A is a schematic diagram of a matrix filter using acoustic resonators.

FIG. 3B is a schematic diagram of a sub-filter of FIG. 3A.

FIG. 4 is a schematic diagram of a matrix filter using transversely-excited film bulk acoustic resonators.

FIG. 5 is plain view of an embodiment of the matrix filter of FIG. 4.

FIG. 6 is a schematic cross-sectional view of the matrix filter of FIG. 5.

FIG. 7 is a graph of input-output transfer functions of an embodiment of the matrix filter of FIGS. 4-6.

FIG. 8 is flow chart of a process for making a matrix filter using transversely-excited film bulk acoustic resonators.

Throughout this description, elements appearing in figures are assigned three-digit or four-digit reference designators, where the two least significant digits are specific to the element and the one or two most significant digit is the figure number where the element is first introduced. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is a new resonator structure for use in acoustic filters for filtering microwave signals. The XBAR is described in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR, which is incorporated herein by reference in its entirety. An XBAR resonator comprises a conductor pattern having an interdigital transducer (IDT) formed on a thin floating layer or diaphragm of a piezoelectric material. The IDT has two busbars which are each attached to a set of fingers and the two sets of fingers are interleaved on the diaphragm over a cavity formed in a substrate upon which the resonator is mounted. The diaphragm spans the cavity and may include front-side and/or back-side dielectric layers. A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm, such that the acoustic energy flows substantially normal to the surfaces of the layer, which is orthogonal or transverse to the direction of the electric field generated by the IDT. XBAR resonators provide very high electromechanical coupling and high frequency capability.

Acoustic filters are typically required to match a system impedance, such as 50 ohms. The system impedance and operating frequency dictate a required equivalent capacitance C₀ for a filter using a conventional ladder circuit. C0 is inversely proportional to frequency. XBAR resonators have low capacitance per unit area compared to other acoustic resonators. Thus, ladder filter circuits using XBAR resonators may be much larger than comparable filters using other types of acoustic resonators.

The following describes a filter circuit architecture that allows low frequency filters to be implemented with small XBAR resonators. It also describes improved XBAR resonators, filters and fabrication techniques that reduce static capacitance in radio frequency filters having sub-filters connected in parallel between two ports where the sub-filters have XBARs on different substrates of different die. The sub-filter XBARs have a piezoelectric plate with a back surface attached to the different substrates and portions of the plate forming diaphragms spanning cavities in the substrates. Interleaved fingers of IDTs are on the diaphragms and the thicknesses of the piezoelectric plate portions may be different.

FIG. 1 shows a simplified schematic top view, orthogonal cross-sectional views, and a detailed cross-sectional view of a transversely-excited film bulk acoustic resonator (XBAR) 100. XBAR resonators such as the resonator 100 may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. XBARs are particularly suited for use in filters for communications bands with frequencies above 3 GHz. The matrix XBAR filters described in this patent are also suited for frequencies above 1 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on a surface of a piezoelectric plate 110 having parallel front and back surfaces 112, 114, respectively. The piezoelectric plate is a thin single-crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back surfaces is known and consistent. The piezoelectric plate may be Z-cut (which is to say the Z axis is normal to the front and back surfaces 112, 114), rotated Z-cut, or rotated YX cut. XBARs may be fabricated on piezoelectric plates with other crystallographic orientations.

The back surface 114 of the piezoelectric plate 110 is attached to a surface of the substrate 120 except for a portion of the piezoelectric plate 110 that forms a diaphragm 115 spanning a cavity 140 formed in the substrate. The portion of the piezoelectric plate that spans the cavity is referred to herein as the “diaphragm” 115 due to its physical resemblance to the diaphragm of a microphone. As shown in FIG. 1, the diaphragm 115 is contiguous with the rest of the piezoelectric plate 110 around all of a perimeter 145 of the cavity 140. In this context, “contiguous” means “continuously connected without any intervening item”. In other configurations, the diaphragm 115 may be contiguous with the piezoelectric plate around at least 50% of the perimeter 145 of the cavity 140.

The substrate 120 provides mechanical support to the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material or combination of materials. The back surface 114 of the piezoelectric plate 110 may be bonded to the substrate 120 using a wafer bonding process. Alternatively, the piezoelectric plate 110 may be grown on the substrate 120 or attached to the substrate in some other manner. The piezoelectric plate 110 may be attached directly to the substrate or may be attached to the substrate 120 via one or more intermediate material layers (not shown in FIG. 1).

“Cavity” has its conventional meaning of “an empty space within a solid body.” The cavity 140 may be a hole completely through the substrate 120 (as shown in Section A-A and Section B-B) or a recess in the substrate 120 under the diaphragm 115. The cavity 140 may be formed, for example, by selective etching of the substrate 120 before or after the piezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigital transducer (IDT) 130. The IDT 130 includes a first plurality of parallel fingers, such as finger 136, extending from a first busbar 132 and a second plurality of fingers extending from a second busbar 134. The first and second pluralities of parallel fingers are interleaved. The interleaved fingers overlap for a distance AP, commonly referred to as the “aperture” of the IDT. The center-to-center distance L between the outermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR 100. A radio frequency or microwave signal applied between the two busbars 132, 134 of the IDT 130 excites a primary acoustic mode within the piezoelectric plate 110. The primary acoustic mode of an XBAR is a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric plate 110, which is also normal, or transverse, to the direction of the electric field created by the IDT fingers. Thus, the XBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that at least the fingers of the IDT 130 are disposed on the diaphragm 115 of the piezoelectric plate which spans, or is suspended over, the cavity 140. As shown in FIG. 1, the cavity 140 has a rectangular shape with an extent greater than the aperture AP and length L of the IDT 130. A cavity of an XBAR may have a different shape, such as a regular or irregular polygon. The cavity of an XBAR may have more or fewer than four sides, which may be straight or curved.

The detailed cross-section view (Detail C) shows two IDT fingers 136 a, 136 b on the surface of the piezoelectric plate 110. The dimension p is the “pitch” of the IDT and the dimension w is the width or “mark” of the IDT fingers. A dielectric layer 150 may be formed between and optionally over (see IDT finger 136 a) the IDT fingers. The dielectric layer 150 may be a non-piezoelectric dielectric material, such as silicon dioxide or silicon nitride. The dielectric layer 150 may be formed of multiple layers of two or more materials. The IDT fingers 136 a and 136 b may be aluminum, copper, beryllium, gold, tungsten, molybdenum, alloys and combinations thereof, or some other conductive material. Thin (relative to the total thickness of the conductors) layers of other metals, such as chromium or titanium, may be formed under and/or over and/or as layers within the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and/or to passivate or encapsulate the fingers and/or to improve power handling. The busbars of the IDT 130 may be made of the same or different materials as the fingers.

For ease of presentation in FIG. 1, the geometric pitch and width of the IDT fingers is greatly exaggerated with respect to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR has more than ten parallel fingers in the IDT 110. An XBAR may have hundreds of parallel fingers in the IDT 110. Similarly, the thickness of the fingers in the cross-sectional views is greatly exaggerated.

An XBAR based on shear acoustic wave resonances can achieve better performance than current state-of-the art surface acoustic wave (SAW), film-bulk-acoustic-resonators (FBAR), and solidly-mounted-resonator bulk-acoustic-wave (SMR BAW) devices. In particular, the piezoelectric coupling for shear wave XBAR resonances can be high (>20%) compared to other acoustic resonators. High piezoelectric coupling enables the design and implementation of microwave and millimeter-wave filters of various types with appreciable bandwidth.

The basic behavior of acoustic resonators, including XBARs, is commonly described using the Butterworth Van Dyke (BVD) circuit model as shown in FIG. 2A. The BVD circuit model consists of a motional arm and a static arm. The motional arm includes a motional inductance L_(m), a motional capacitance C_(m), and a resistance R_(m). The static arm includes a static capacitance C₀ and a resistance R₀. While the BVD model does not fully describe the behavior of an acoustic resonator, it does a good job of modeling the two primary resonances that are used to design band-pass filters, duplexers, and multiplexers (multiplexers are filters with more than 2 input or output ports with multiple passbands).

The first primary resonance of the BVD model is the motional resonance caused by the series combination of the motional inductance L_(m) and the motional capacitance C_(m). The second primary resonance of the BVD model is the anti-resonance caused by the combination of the motional inductance L_(m), the motional capacitance C_(m), and the static capacitance C₀. In a lossless resonator (R_(m)=R₀=0), the frequency F_(r) of the motional resonance is given by

$\begin{matrix} {F_{r} = \frac{1}{2\pi\sqrt{L_{m}C_{m}}}} & (1) \end{matrix}$

The frequency F_(a) of the anti-resonance is given by

$\begin{matrix} {F_{a} = {F_{r}\sqrt{1 + \frac{1}{\gamma}}}} & (2) \end{matrix}$

where γ=C₀/C_(m) is dependent on the resonator structure and the type and the orientation of the crystalline axes of the piezoelectric material.

FIG. 2B is a graph 200 of the magnitude of admittance of a theoretical lossless acoustic resonator. The acoustic resonator has a resonance 212 at a resonance frequency where the admittance of the resonator approaches infinity. The resonance is due to the series combination of the motional inductance L_(m) and the motional capacitance C_(m) in the BVD model of FIG. 2A. The acoustic resonator also exhibits an anti-resonance 214 where the admittance of the resonator approaches zero. The anti-resonance is caused by the combination of the motional inductance L_(m), the motional capacitance C_(m), and the static capacitance C₀. In a lossless resonator (R_(m)=R₀=0), the frequency F_(r) of the resonance is given by

$\begin{matrix} {F_{r} = \frac{1}{2\pi\sqrt{L_{m}C_{m}}}} & (1) \end{matrix}$

The frequency F_(a) of the anti-resonance is given by

$\begin{matrix} {F_{a} = {F_{r}\sqrt{1 + \frac{1}{\gamma}}}} & (2) \end{matrix}$

In over-simplified terms, the lossless acoustic resonator can be considered a short circuit at the resonance frequency 212 and an open circuit at the anti-resonance frequency 214. The resonance and anti-resonance frequencies in FIG. 2B are representative, and an acoustic resonator may be designed for other frequencies.

FIG. 2C shows the circuit symbol for an acoustic resonator such as an XBAR. This symbol will be used to designate each acoustic resonator in schematic diagrams of filters in subsequent figures.

FIG. 3A is a schematic diagram of a matrix filter 300 using acoustic resonators. The matrix filter 300 includes an array 310 of n sub-filters 320-1, 320-2, 320-n connected in parallel between a first filter port (FP1) and a second filter port (FP2), where n is an integer greater than one. Each of the n sub-filters 320-1, 320-2, 320-n is a bandpass filter having a bandwidth about 1/n times the bandwidth of the matrix filter 300. The sub-filters 320-1, 320-2, 320-n have contiguous passbands such that the bandwidth of the matrix filter 300 is equal to the sum of the bandwidths of the constituent sub-filters. In the subsequent examples in this patent n=3. n can be less than or greater than 3 as necessary to provide the desired bandwidth for the matrix filter 300. In some cases, the n sub-filters 320-1, 320-2, 320-n may include one or more XBARs. The filter 300 and/or sub-filters may be RF filters that pass frequency bands defined by the 5G NR standard.

The array 310 of sub-filters is terminated at the FP1 end by acoustic resonators XL1 and XH1, which are preferably but not necessarily XBARs. The array 310 of sub-filters is terminated at the FP2 end by acoustic resonators XL2 and XH2, which are preferably but not necessarily XBARs. The acoustic resonators XL1, XL2, XH1, and XH2 create “transmission zeros” at their respective resonance frequencies. A “transmission zero” is a frequency where the input-output transfer function of the filter 300 is very low (and would be zero if the acoustic resonators XL1, XL2, XH1, and XH2 were lossless). The zero transmission may be caused by one or more of the acoustic resonators creating a very low impedance to ground and thus, in this configuration cause the sub-filters to be removed as filtering components as the acoustic resonators are basically short circuits to ground so that the sub-filters have no effect on the filter 300 during transmission zero frequencies. Typically, but not necessarily, the resonance frequencies of XL1 and XL2 are equal, and the resonance frequencies of XH1 and XH2 are equal. The resonant frequencies of the acoustic resonators XL1, XL2 are selected to provide transmission zeros adjacent to the lower edge of the filter passband. XL1 and XL2 may be referred to as “low-edge resonators” since their resonant frequencies are proximate the lower edge of the filter passband. The acoustic resonators XL1 and XL2 also act as shunt inductances to help match the impedance at the ports of the filter to a desired impedance value. In the subsequent examples in this patent, the impedance at all ports of the filters is matched to 50 ohms. The impedance may be another value if desired, such as 20, 100 or 1000 ohms. The resonant frequencies of acoustic resonators XH1, XH2 are selected to provide transmission zeros at or above the higher edge of the filter passband. XH1 and XH2 may be referred to as “high-edge resonators” since their resonant frequencies are proximate the higher edge of the filter passband. High-edge resonators XH1 and XH2 may not be required in all matrix filters, such as filters where high rejection above the passband is not required.

FIG. 3B is a schematic diagram of a sub-filter 350 suitable for each of sub-filters 320-1, 320-2, and 320-n of filter 300. The sub-filter 350 includes three acoustic resonators XA, XB, XC connected in series between a first sub-filter port (SP1) which can be connected to FP1 and a second sub-filter port (SP2) which can be connected to FP2. The acoustic resonators XA, XB, XC are preferably but not necessarily XBARs. The sub-filter 350 includes two coupling capacitors CA, CB, each of which is connected between ground and a respective node between two of the acoustic resonators. The inclusion of three acoustic resonators in the sub-filter 350 is exemplary. A sub-filter may have m acoustic resonators, where m is an integer greater than one. A sub-filter with m acoustic resonators includes m−1 coupling capacitors The m acoustic resonators of a sub-filter are connected in series between the two ports SP1 and SP2 of a sub-filter and each of the m−1 coupling capacitors is connected between ground and a node between a respective pair of acoustic resonators from the in acoustic resonators.

Compared to other types of acoustic resonators, XBARs have very high electromechanical coupling (which results in a large difference between the resonance and anti-resonance frequencies), but low capacitance per unit area. The matrix filter architecture, as shown in FIG. 3A and FIG. 3B, takes advantage of the high electromechanical coupling of XBARs without requiring high resonator capacitance. Thus, this architecture improves high frequency bandpass filtering by passing a wider range of high frequency without requiring processing to form or the space to form high-edge resonators XH1 and XH2.

FIG. 4 is a schematic circuit diagram of an exemplary matrix filter 400 implemented with XBARs. The matrix filter 400 includes three sub-filters 420-1, 420-2, 420-3 connected in parallel between a first filter port (FP1) and a second filter port (FP2). The sub-filters 420-1, 420-2, 420-3 have contiguous passbands such that the bandwidth of the matrix filter 300 is equal to the sum of the bandwidths of the constituent sub-filters. Each sub-filter includes three XBARs connected in series and two coupling capacitors. For example, sub-filter 420-1 includes series XBARs X1A, X1B, and X1C and two coupling capacitors C1A, C1B each of which is connected between ground and a respective node between two of the acoustic resonators. Components of the other sub-filters 420-2 and 420-3 are similarly identified using 2's and 3's as those using 1's in sub-filter 420-1. Low-edge XBARs XL1 and XL2 are connected between FP1 and FP2, respectively, and ground. All of the capacitors within the sub-filters are connected to ground through a common inductor L1. The inclusion of the inductor L1 improves the out-of-band rejection of the matrix filter 400 which improves filtering. The matrix filter 400 does not include high-edge resonators.

The exemplary matrix filter 400 is symmetrical in that the impedances at FP1 and FP2 are both equal to 50 ohms. The impedance may be another value if desired, such as 20, 100 or 1000 ohms. The internal circuitry of the filter is also symmetrical, with XBARs X_A and X_C within each sub-filter being the same and low-edge resonators XL1 and XL2 being the same. Other matrix filters may be designed to have significantly different impedances at FP1 and FP2, in which event the internal circuitry will not be symmetrical.

FIG. 5 is a plan view of an exemplary matrix filter 500 which has the same schematic circuit diagram as the matrix filter 400 of FIG. 4. The exemplary matrix filter is an LTE band 41 bandpass filter with a passband from 2496 to 2690 MHz.

The matrix filter 500 includes three Z-cut lithium tantalate piezoelectric plate thickness portions 510-3 for filter 410-3, 510-1 for filter 410-1, and 510-2 for filter 410-2. The portions 510-1, 2 and 3 may be three different piezoelectric diaphragm thicknesses. Any number of portions 510-1, 2 and 3 can be on any number of plates; and any number of those plates can be on any number of substrates of different die or chips. Other matrix filters may use lithium niobate piezoelectric plates and other crystal orientations including rotated Z-cut and rotated Y-cut.

For example, the portions 510-1, 2 and 3 could be three separate plates each having a different thickness, or three portions of the same plate having different thicknesses created using a process multiple thicknesses on the same plate. Such a process may thin portions 510-2 and 3 from the thickness of portion 510-1; and then further thin portion 510-3 from the thickness of portion 510-2.

The back surface of each plate is attached to one substrate. In another case it is attached to more than one substrate. The back surface of each of portions 510-1, 2 and 3 is bonded to a substrate. Whether or not they are of the same or of separate plates, in a first case, the portions 510-1, 2 and 3 are each bonded to a separate substrate of a different die (substrates and die not visible in FIG. 5). In a second case, two of portions 510-1, 2 and 3 are bonded to one substrate of one die; and the other portion is bonded to a second substrate of a separate die. In a third case, all three portions 510-1, 2 and 3 are bonded to a single plate which is bonded to a single substrate of a single die.

The thickness of the piezoelectric plate portion 510-3 between its front and back surfaces is the thinnest of all three portions 510-1, 2 and 3. The thickness of portion 510-1 is the thickest and that of portion 510-2 is in between that of the thickness of other two portions. The thickness of plate portion 510-3 is 730 nm. The thickness of plate portion 510-2 is 744 nm. The thickness of plate portion 510-1 is 762 nm. Each of these three thicknesses may be plus or minus 10 nm.

The low-edge resonators XL1 and XL2 may be formed using or on the thickest plate portion, such as portion 510-1. Any high-edge resonator will be formed using or on the thinnest plate portion, such as portion 510-3.

The matrix filter 500 includes eleven XBARs, such as the XBAR 520. A cavity (not visible) is formed in the substrates under each XBAR. Each XBAR is shown as a rectangle with vertical hatching and is identified by the designator (XL1, X1A, . . . ) used in the schematic diagram of FIG. 4. The vertical hatching is representative of the direction of the IDT fingers of each XBAR but not to scale. Each XBAR has between 65 and 130 IDT fingers, one of which is shown as finger 536. The IDT fingers are aluminum and 925 nm thick. The apertures AP (vertical direction as shown in FIG. 5) of the overlap of the interleaved fingers of the XBARs range from 40 microns to 58 microns, and the lengths L (left-right direction as shown in FIG. 5) range from 500 to 1000 microns. In other embodiments of XBAR matrix filters, the XBARs may be divided into sections to limit the length of the diaphragm within each XBAR. The pitch p of the IDTs of each XBAR is between 7.5 and 8.6 microns and the mark/pitch ratio of each XBAR is between 0.22 and 0.31.

The XBARs are connected to each other by conductors such as conductor 530 that may also be formed on and connect between the substrates. Cross-hatched rectangles are metal-insulator-metal capacitors used as the sub-filter coupling capacitors in this example of FIG. 4, of which only capacitors 540 and 542 are identified. The identified capacitors 540 and 542 are C3A and C3B, respectively, in the schematic diagram of FIG. 4. The other sub-filter coupling capacitors C1A-B and C2A-B for filters 410-1 and 410-2, respectively, are connected similarly to capacitors 540 and 542. The sub-filter coupling capacitors C1A-B, C2A-B and C3A-B are shown formed on and/or from the same plate portion 510-1, 2 or 3 of the corresponding filter 410-1, 2 or 3, respectively, such as shown in the figure. However, the coupling capacitors may be formed on any of the plate portions 510-1, 2 or 3; or on separate portions of any of the substrates. The capacitors C1A-B, C2A-B and C3A-B may be formed on the substrates 420-1, 2 or 3, respectively. The coupling capacitors may be separate from the substrates of portions 510, such as by being discrete components or formed on a circuit card used to interconnect the resonators.

Connections from the filter 510 and circuitry external to the filter are made by means of conductive pads indicated by shaded circles, such as conductive pad 550. The conductive pads for Filter Port 1 (FP1), Filter Port 2 (FP2), and ground (GND) are labeled. The three other conductive pads L11, L21 and L31 are connect to ground through inductor L1 (in FIG. 4), which is located external to the filter 510. The conductor pads may be connected using solder bumps or other connections to the pads. The conductor pads may be formed on and/or be connect between the substrates.

As previously described, the sub-filters of a matrix filter have contiguous passbands that span the passband of the matrix filter. Within a matrix filter, the center frequency of the passband of each sub-filter is different from the center frequency of any other sub-filter. Consequentially, the resonance frequencies of the XBARs in one sub-filter are different from the resonance frequencies of the XBARs within any other sub-filter.

The resonance frequency of an XBAR is primarily determined by the thickness of the diaphragm or piezoelectric plate portion of the diaphragm within the XBAR. The resonance frequency has a smaller dependence on IDT pitch and mark or finger width. U.S. Pat. No. 10,491,291 describes the use of a dielectric layer formed between the IDT fingers to adjust the resonance frequency of an XBAR. U.S. Pat. No. 10,998,877 describes the use of the plate diaphragm portion thicknesses to adjust the resonance frequency of an XBAR.

FIG. 6 is a schematic cross-section view of the matrix filter 500 at a section plane D-D defined in FIG. 5. The section plane D-D passes through one XBAR (X3A, X1A, X2A) from each of the three sub-filters in the matrix filter 500. Each XBAR includes interleaved IDT fingers (of which only IDT finger 630-1, 2 or 3 is identified) formed on a respective diaphragm spanning a respective cavity in a substrate 620-1, 2 or 3 or a die (not visible). Substrates 620-1, 2 and 3 may be one same, two different or three different substrates. Each substrate 620-1, 2 or 3 may have other components of filter 500. Each diaphragm includes a different piezoelectric plate portion 510-1, 2 or 3 having a different thickness between its front and back surfaces. Each of the front and back surfaces may be planar or flat across the entire surface of the plate or plate portion 510-1, 2 or 3. The back surface of each plate portion may be attached to one or more substrates of one or more die. As in previous figures, the thickness of the piezoelectric plate portions 510-1, 2 or 3 and the thickness, pitch, and finger width of the IDTs are greatly exaggerated for visibility. Drawn to scale, the thickness of the piezoelectric plate portions 10-1, 2 or 3 and the IDT fingers could be less than one-half percent of the thickness of the substrate 620-1, 2 or 3 and each IDT would have 65 to 130 IDT fingers.

The three detail views illustrate the use of piezoelectric plate portion thickness to set the resonance frequencies of the XBARs within each sub-filter. Consider first the detail view of an IDT finger of XBAR X1A (the middle view of the three detail views), which shows an IDT finger 630-1 formed on a portion of the piezoelectric plate portion 510-1. The IDT finger 630-1 is shown with a trapezoidal cross-section. The trapezoidal shape is exemplary and IDT fingers may have other cross-sectional shapes. The piezoelectric plate portion 510-1 has thickness tp1 extending between its front surface 611-1 and its back surface 612-1. Similarly, the right-hand detail shows piezoelectric plate portion 510-2 having thickness tp2 extending between its front surface 611-2 and its back surface 612-2. The left-hand detail shows piezoelectric plate portion 510-3 having thickness tp3 extending between its front surface 611-3 and its back surface 612-3. Each of thicknesses tp1, tp2 and tp3 is different than any of the others.

In this example, XBAR X1A is an element of the sub-filter with the lowest passband frequency and XBAR X3A is an element of the sub-filter with the highest passband frequency. In this case tp1>tp2>tp3≥0. In other cases, two of the thickness tp1, tp2 and tp3 are the same but the third thickness is greater than or less than those two thicknesses. In some cases, there may be only two tp thicknesses, and in other cases there may be more than three tp thicknesses.

Further in this example, XBARs X1B and X1C are also formed on portion 510-1 with XBAR X1A as elements of the sub-filter 410-1 with the lowest passband frequency. XBARs X3B and X3C are also formed on portion 510-3 with XBAR X3A as elements of the sub-filter 410-3 with the highest passband frequency. Finally, XBAR X2B and X2C are also formed on portion 510-2 with XBAR X2A as elements of the sub-filter 410-2 with the passband frequency between that of filters 410-1 and 410-3.

In a more general case where a matrix filter has n sub-filters, which are numbered in order of increasing passband frequency, tp1>tp2> . . . >tpn, where tpi is the thickness of the piezoelectric plate portion extending between its front and a back surfaces of sub-filter i.

The low-edge resonators XL1 and XL2 may be formed using the thickest of the plate portions 510-1, 2 or 3. The low-edge resonators XL1 and XL2 may have their resonance frequencies set by the thickness of the plate portion they are formed using. In addition, or independently, the low-edge resonators XL1 and XL2 may have their resonance frequencies set by a thickness of a top layer dielectric. In this case, the space between IDT fingers of XL1 and XL2 and adjacent IDT fingers (and optionally the IDT finger) would be covered by a dielectric layer having a thickness td1 and td2 to set the XL1 and XL2 resonance frequencies. The dielectric layers may be silicon dioxide, silicon nitride, aluminum oxide or some other dielectric material or combination of materials. The dielectric layers may be the same or different materials.

An XBAR filter device typically includes a passivation dielectric layer applied over the entire surface of the device, other than contact pads, to seal and passivate the conductor patterns and other elements of the device.

FIG. 7 is graph 700 of an estimated performance of a matrix filter similar to the matrix filter 500. The curve 710 is a plot of S21, the input-output transfer function, of the filter determined by estimation of a physical model of the filter. The broken lines 720 mark the band edges of 5G NR communication band n41. The matrix filter architecture extends the application of XBARs to lower frequency communications bands that are impractical using a conventional ladder filter architecture.

The concepts described above for FIGS. 5-7 may be applied to form filters such as filter 500 but with different ranges and/or center points of the pass band. For example, a filter similar to filter 500 may be built with broken lines 720 that mark the band edges of 5G NR communication band n40 or n46.

Description of Methods

FIG. 8 is a simplified flow chart showing a process 800 for making an XBAR or a filter incorporating XBARs. The process 800 starts at 805 with a substrate and a plate of piezoelectric material and ends at 895 with a completed XBAR or filter. It may be an example for forming any of the XBARs herein with any of the piezoelectric plate portions 610-1, 2 or 3 of one or more piezoelectric plates one any one or more of substrates 620-1, 2 or 3. The flow chart of FIG. 8 includes only major process steps. Various conventional process steps (e.g. surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, between, after, and during the steps shown in FIG. 8.

The flow chart of FIG. 8 captures three variations of the process 800 for making an XBAR which differ in when and how cavities are formed in the substrate. The cavities may be formed at steps 810A, 810B, or 810C. Only one of these steps is performed in each of the three variations of the process 800.

The piezoelectric plate may be, for example, Z-cut lithium niobate or lithium tantalate with Euler angles 0, 0, 90°. The piezoelectric plate may be rotated Z-cut lithium niobate with Euler angles 0, β, 90°, where β is in the range from −15° to +5°. The piezoelectric plate may be rotated Y-cut lithium niobate or lithium tantalate with Euler angles 0, β, 0, where β is in the range from 0 to 60°. The piezoelectric plate may be some other material or crystallographic orientation. The substrate may preferably be silicon. The substrate may be some other material that allows formation of deep cavities by etching or other processing.

In one variation of the process 800, one or more cavities are formed in the substrate at 810A, before the piezoelectric plate is bonded to the substrate at 820. A separate cavity may be formed for each resonator in a filter device. The one or more cavities may be formed using conventional photolithographic and etching techniques. Typically, the cavities formed at 810A will not penetrate through the substrate.

At 820, the piezoelectric plate is bonded to the substrate. Bonding at 820 may be bonding any of the piezoelectric plates 610-1, 2 or 3 to substrate 620-1, 2 or 3. The piezoelectric plate and the substrate may be bonded by a wafer bonding process. Typically, the mating surfaces of the substrate and the piezoelectric plate are highly polished. One or more layers of intermediate materials, such as an oxide or metal, may be formed or deposited on the mating surface of one or both of the piezoelectric plate and the substrate. One or both mating surfaces may be activated using, for example, a plasma process. The mating surfaces may then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and the substrate or intermediate material layers.

A conductor pattern, including IDTs of each XBAR, is formed at 830 by depositing and patterning two or more conductor levels on the front side of the piezoelectric plate. The conductor levels typically include a first conductor level that includes the IDT fingers, and a second conductor level formed over the IDT busbars and other conductors except the IDT fingers. In some devices, a third conductor levels may be formed on the contact pads. Each conductor level may be one or more layers of, for example, aluminum, an aluminum alloy, copper, a copper alloy, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below (i.e. between each conductor layer and the piezoelectric plate) and/or on top of each conductor layer. For example, a thin film of titanium, chrome, or other metal may be used to improve the adhesion between the first conductor level and the piezoelectric plate. The second conductor level may be conduction enhancement layer of gold, aluminum, copper or other higher conductivity metal may be formed over portions of the first conductor level (for example the IDT bus bars and interconnections between the IDTs).

Each conductor level may be formed at 830 by depositing the appropriate conductor layers in sequence over the surface of the piezoelectric plate. The excess metal may then be removed by etching through patterned photoresist. The conductor level can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, and other etching techniques.

Alternatively, each conductor level may be formed at 830 using a lift-off process. Photoresist may be deposited over the piezoelectric plate. and patterned to define the conductor level. The appropriate conductor layers may be deposited in sequence over the surface of the piezoelectric plate. The photoresist may then be removed, which removes the excess material, leaving the conductor level.

When a conductor level has multiple layers, the layers may be deposited and patterned separately. In particular, different patterning processes (i.e. etching or lift-off) may be used on different layers and/or levels and different masks are required where two or more layers of the same conductor level have different widths or shapes.

At 840, dielectric layers may be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. As previously described, the dielectric layers may include a different dielectric thickness over the IDT fingers of the XBARs within each sub-filter. Each dielectric layer may be deposited using a conventional deposition technique such as sputtering, evaporation, or chemical vapor deposition. Each dielectric layer may be deposited over the entire surface of the piezoelectric plate, including on top of the conductor pattern. Alternatively, one or more lithography processes (using photomasks) may be used to limit the deposition of the dielectric layers to selected areas of the piezoelectric plate, such as only between the interleaved fingers of the IDTs. Masks may also be used to allow deposition of different thicknesses of dielectric materials on different portions of the piezoelectric plate.

The matrix filter shown in FIG. 5 and FIG. 6 includes metal-insulator-metal (MIM) capacitors. A MIM capacitor consists of a first metal level and a second metal level separated by a dielectric layer. When a matrix filter includes MIM capacitor, the steps of forming the conductor patterns at 830 and forming the dielectric layers at 840 must overlap. At least one dielectric layer has to be formed at 840 after a first metal level is formed at 830 and before a final metal level is formed at 830. The MIM capacitors may be formed beside or on any of the piezoelectric plate portions 610-1, 2 or 3; or substrates 620-1, 2 or 3.

In a second variation of the process 800, one or more cavities are formed in the back side of the substrate at 810B. A separate cavity may be formed for each resonator in a filter device. The one or more cavities may be formed using an anisotropic or orientation-dependent dry or wet etch to open holes through the back side of the substrate to the piezoelectric plate. In this case, the resulting resonator devices will have a cross-section as shown in FIG. 1.

In the second variation of the process 800, a back-side dielectric layer may be formed at 850. In the case where the cavities are formed at 810B as holes through the substrate, the back-side dielectric layer may be deposited through the cavities using a conventional deposition technique such as sputtering, evaporation, or chemical vapor deposition.

In a third variation of the process 800, one or more cavities in the form of recesses in the substrate may be formed at 810C by etching the substrate using an etchant introduced through openings in the piezoelectric plate. A separate cavity may be formed for each resonator in a filter device.

In all variations of the process 800, the filter device is completed at 860. Actions that may occur at 860 include depositing an encapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or a portion of the device; forming bonding pads or solder bumps or other means for making connection between the device and external circuitry; excising individual devices from a wafer containing multiple devices; other packaging steps; and testing. Another action that may occur at 860 is to tune the resonant frequencies of the resonators within the device by adding or removing metal or dielectric material from the front side of the device. After the filter device is completed, the process ends at 895.

The descriptions herein such as for FIGS. 4-8 provide improved XBAR radio frequency filter configurations by using XBAR sub-filters with different thicknesses of the piezoelectric plate portions connected in parallel between two ports. The sub-filters have a piezoelectric plate with a back surface attached to the different substrates and portions of the plate forming diaphragms spanning cavities in the substrates. Interleaved fingers of IDTs are on the diaphragms. The different thickness portions of the piezoelectric plates may be on different substrates of different die connected in parallel between two ports.

These configurations form a distributed (matrix) XBAR filter that allows for the reduction of required resonator static capacitance C0, and therefore a reduction in required die area. These configurations are also scalable to arbitrary order and can readily be made reconfigurable with the use of RF switches. By incorporating the configurations' multi-die approach, similar to a ‘split ladder’ topology, additional freedom in the design of the distributed filter is achieved.

Without these configurations, constraining all resonators of multiple sub-filters to a single die requires frequency separation of resonators to be achieved by varying top layer oxide and/or electrode dimensions. Instead, the multi-die configurations introduce the membrane thickness as an additional degree of freedom that may be applied by sub-filter resonator groups.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items. 

It is claimed:
 1. A method of forming a radio frequency filter, comprising: connecting a first sub-filter and a second sub-filter in parallel between a first port and a second port, wherein connecting each of the first and second sub-filters comprises: bonding to a substrate a back surface of a piezoelectric plate having front and back surfaces, portions of the piezoelectric plate forming a plurality of diaphragms spanning respective cavities in the substrate; and forming a conductor pattern on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of resonators, interleaved fingers of each IDT disposed on a respective diaphragm of the plurality of diaphragms, wherein a thickness of the portion of the piezoelectric plate of the first sub-filter is different from a thickness of the portion of the piezoelectric plate of the second sub-filter.
 2. The method of claim 1, wherein the piezoelectric plate of the first sub-filter is the same as the piezoelectric plate of the second sub-filter.
 3. The method of claim 1, wherein the piezoelectric plate of the first sub-filter is different than the piezoelectric plate of the second sub-filter.
 4. The method of claim 1, wherein the substrate of the first sub-filter is different than the substrate of the second sub-filter.
 5. The method of claim 1, wherein: the first and second portions and the first and second IDTs are configured such that radio frequency signals applied to the first and second IDTs excite primary shear acoustic modes in the first and second portions of the piezoelectric plates forming the plurality of diaphragms spanning the respective cavities in the different substrates; and the thicknesses of the first and second portions are selected to tune the primary shear acoustic modes in the first and second portions.
 6. The method of claim 1, further comprising: connecting a third sub-filter in parallel between the first port and the second port, wherein connecting the third sub-filter comprises: bonding to a substrate a back surface of a piezoelectric plate having front and back surfaces, portions of the piezoelectric plate forming a plurality of diaphragms spanning respective cavities in the substrate; and forming a conductor pattern on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of resonators, interleaved fingers of each IDT disposed on a respective diaphragm of the plurality of diaphragms, wherein a thickness of the piezoelectric plate portion of the third sub-filter is different from a thickness of the piezoelectric plate portion of the first and portion of the second sub-filters.
 7. The method of claim 6, wherein the thickness of the piezoelectric plate portion of the first sub-filter is a thickness of between 720 nm and 740 nm extending between a front surface and a back surface of the piezoelectric plate portion of the first sub-filter; wherein the thickness of the piezoelectric plate portion of the second sub-filter is a thickness of between 752 nm and 772 nm extending between a front surface and a back surface of the piezoelectric plate portion of the second sub-filter; and wherein the thickness of the piezoelectric plate portion of the third sub-filter is a thickness of between 734 nm and 754 nm extending between a front surface and a back surface of the piezoelectric plate portion of the third sub-filter.
 8. The method of claim 6, wherein connecting each of the three sub-filters further comprises: connecting three resonators in series between the first port and the second port; and connecting two coupling capacitors between ground and a respective node between two of the resonators of the sub-filter.
 9. The method of claim 8, further comprising: connecting a first low-edge resonator, from the plurality of resonators, between the first port and ground; connecting a second low-edge resonator, from the plurality of resonators, between the second port and ground; wherein respective resonance frequencies of the first and second low-edge resonators are adjacent to a lower edge of a passband of the filter.
 10. The method of claim 9, wherein: two of the resonators of each sub-filter are symmetrical in response; the low-edge resonators have the same response; each of the coupling capacitors is a metal-insulator-metal capacitor; and the sub-filters and the low-edge resonators form a matrix filter having a contiguous passband formed by passbands of the sub-filters; and a center frequency of a passband of each sub-filter is different from a center frequency of any other sub-filter.
 11. The method of claim 6, wherein: the thickness of the piezoelectric plate portion of the first sub-filter is thinner than the thickness of the piezoelectric plate portion of the second sub-filter; and the thickness of the piezoelectric plate portion of the second sub-filter is thinner than the thickness of the piezoelectric plate portion of the third sub-filter.
 12. A method of forming a radio frequency filter, comprising: connecting a first sub-filter and a second sub-filter in parallel between a first port and a second port, wherein connecting the first sub-filter comprises: bonding to a first substrate a first back surface of a first piezoelectric plate having first front and back surfaces, first portions of the first piezoelectric plate forming a first plurality of diaphragms spanning respective first cavities in the first substrate; and forming a first conductor pattern on the first front surface, the first conductor pattern including a first plurality of interdigital transducers (IDTs) of a first respective plurality of resonators, first interleaved fingers of each IDT disposed on a first respective diaphragm of the first plurality of diaphragms, wherein connecting the second sub-filters comprises: bonding to a second substrate a second back surface of a second piezoelectric plate having second front and back surfaces, second portions of the second piezoelectric plate forming a second plurality of diaphragms spanning respective second cavities in the second substrate; and forming a second conductor pattern on the second front surface, the second conductor pattern including a second plurality of interdigital transducers (IDTs) of a second respective plurality of resonators, second interleaved fingers of each IDT disposed on a second respective diaphragm of the second plurality of diaphragms, wherein a thickness of the first piezoelectric plate first portions are thicker than a thickness of the second piezoelectric plate second portions, and wherein the first substrates is different than the second substrate.
 13. The method of claim 12, wherein the first piezoelectric plate is the same as the second piezoelectric plate.
 14. The method of claim 12, wherein the first piezoelectric plate is different than the second piezoelectric plate.
 15. The method of claim 12, wherein: the first and second portions and the first and second IDTs are configured such that radio frequency signals applied to the first and second IDTs excite primary shear acoustic modes in the first and second portions of the first and second piezoelectric plate portions forming the first and second plurality of diaphragms spanning the first and second respective cavities in the first and second different substrates; and the thicknesses of the first and second portions are selected to tune the primary shear acoustic modes in the first and second portions.
 16. The method of claim 12, wherein connecting each of the first and second sub-filters further comprises: connecting three resonators connected in series between the first port and the second port; connecting each of two coupling capacitors between ground and a respective node between two of the resonators of the sub-filter; and further comprising: connecting a first low-edge resonator, from the plurality of resonators, between the first port and ground; connecting a second low-edge resonator, from the plurality of resonators, between the second port and ground; wherein respective resonance frequencies of the first and second low-edge resonators are adjacent to a lower edge of a passband of the filter.
 17. A method of forming radio frequency filter, comprising: connecting a first sub-filter, a second and a third sub-filter in parallel between a first port and a second port, wherein connecting each of the first, second and third sub-filters comprises: bonding to a substrate a back surface of a piezoelectric plate having front and back surfaces, portions of the piezoelectric plate forming a plurality of diaphragms spanning respective cavities in the substrate; and forming a conductor pattern on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of resonators, interleaved fingers of each IDT disposed on a respective diaphragm of the plurality of diaphragms, wherein a thickness of the piezoelectric plate portion of the first sub-filter is different from a thickness of the piezoelectric plate portion of the second and third sub-filter, and wherein a thickness of the piezoelectric plate portion of the second sub-filter is different from a thickness of the piezoelectric plate portion of the third sub-filter.
 18. The method of claim 17, wherein the piezoelectric plate of the first sub-filter different than the piezoelectric plate of the second sub-filter and the third sub-filter, and wherein the piezoelectric plate of the second sub-filter is different than the piezoelectric plate of the third sub-filter.
 19. The method of claim 18, wherein the substrate of the first sub-filter different than the substrate of the second sub-filter and the third sub-filter, and wherein the substrate of the second sub-filter is different than the substrate of the third sub-filter.
 20. The method of claim 17, wherein: the piezoelectric plate portions and the IDTs of the first, second and third sub-filter are configured such that radio frequency signals applied to the IDTs excite primary shear acoustic modes in the portions of the piezoelectric plate portions forming the plurality of diaphragms spanning the respective cavities in the different substrates; and the thicknesses of the piezoelectric plate portions of the first, second and third sub-filters are selected to tune the primary shear acoustic modes in the portions of the piezoelectric plates. 